Electrical apparatus for initiating combustion in free piston engines

ABSTRACT

Electrical apparatus is disclosed for initiating and controlling combustion in free piston engines having a power piston freely reciprocating within a power cylinder of the engine and alternately expanding and compressing gases in a combustion chamber within the power cylinder. A velocity probe in the form of a magnet arranged to move with the power piston and a sensing coil fixed to the housing of the free piston engine provides an electrical signal indicating the instantaneous value of the velocity of the power piston of the free piston engine. This electrical signal from the velocity probe is applied to a Schmitt trigger through a suitable electrical connection such as a voltage divider network or preferably a resistor-capacitor and diode network, and the Schmitt trigger provides an electrical timing signal to conventional means for initiating successive combustion power strokes within the engine. Using the electrical apparatus of the present invention, combustion is initiated for each power stroke at a point in time related to the velocity of the power piston, and thus to the time at which the power piston reverses direction, without the necessity of regard to the physical position, i.e., linear location, of the power piston along its path of movement within the power cylinder. Specifically, the Schmitt trigger is set in one condition when the piston velocity increases to a predetermined level as the piston accelerates during the first part of its movement toward a firing position. Then, as the piston slows down while approaching its reversal for the start of the power stroke, the decrease in piston velocity to a predetermined level sets the Schmitt trigger in another condition to cause a spark igniting another combustion initiating action. Additional electrical circuitry is included which may be used to aid in starting free piston engines.

United States Patent 15] 3,673,999 Lacy et al. July 4, 1972 I541ELECTRICAL APPARATUS FOR 57 ABSTRACT lNlTlATlNG COMBUSTION IN, FREEPISTON ENGINES [72] Inventors: James G. Lacy; John V. Byrne, both ofDublin, Ireland [73] Assignee: Anton Braun, Minneapolis, Minn.

22 Filed: Aug. 24, 1970 [2]] Appl. No.: 66,215

Primary Examiner-Laurence M. Goodridge Attorney-Frederick E. Lange andWilliam C. Babcock Electrical apparatus is disclosed for initiating andcontrolling combustion in free piston engines having a power pistonfreely reciprocating within a power cylinder of the engine and alternately expanding and compressing gases in a combustion chamber withinthe power cylinder. A velocity probe in the form of a magnet arranged tomove with the power piston and a sensing coil fixed to the housing ofthe free piston engine provides an electrical signal indicating theinstantaneous value of the velocity of the power piston of the freepiston engine. This electrical signal from the velocity probe is appliedto a Schmitt trigger through a suitable electrical connection such as avoltage divider network or preferably a resistor-capacitor and diodenetwork, and the Schmitt trigger provides an electrical timing signal toconventional means for initiating successive combustion power strokeswithin the engine. Using the electrical apparatus of the presentinvention, combustion is initiated for each power stroke at a point intime related to the velocity of the power piston, and thus to the timeat which the power piston reverses direction, without the necessity ofregard to the physical position, i.e., linear location, of the powerpiston along its path of movement within the power cylinder.Specifically, the Schmitt trigger is set in one condition when thepiston velocity increases to a predetermined level as the pistonaccelerates during the first part of its movement toward a firingposition Then, as the piston slows down while approaching its reversalfor the start of the power stroke, the decrease in piston velocity to apredetermined level sets the Schmitt trigger in another condition tocause a spark igniting another combustion initiating action. Additionalelectrical circuitry is included which may be used to aid in startingfree piston engines.

11 Claims, 17 Drawing Figures PATEIITEDJUL 4 m2 SHEET 3 [IF 4 VOLTAGEACROSS TERMINALS 30 AND 32 T-2 VOLTAGE AT COLLECTOR 76 T-z VOLTAGE ATCOLLECTOR 92 FIE l AST- 4 FIE 12 I INVENTOR$ ddMES a. may, doll/V 1/.BYE/V5 PNENTEnJum mm 9.979 999 SHEET 4 OF 4 2 42 I f -lO 1 I 172 57 l IF\ I 52 -/17 Z 44 170 85 172 g s l nnn'mm m MEAN VALUE OF VOLTAGE ON c186 184 ENGINE STARTING Y FROM REST INVENTORS' c/AIMESGLACM c/OH/V IfBVQA/E ELECTRICAL APPARATUS FOR INITIATING COMBUSTION IN FREE PISTONENGINES CROSS REFERENCES The present invention provides improved meansfor practicing the invention disclosed and claimed in an applicationSer. No. 066,385, entitled FREE PISTON ENGINE IGNITION APPARATUS filedof even date herewith by Anton Braun.

BACKGROUND Combustion is initiated by many techniques in prior art freepiston engines. The most often used technique is where a directmechanical link is made to a piston and combustion is initiated by meansof this mechanical link at a predetermined linear location of the pistonalong its normal path of movement. Examples of this technique are wherecombustion initiating apparatus is directly geared to the pistonmovement or where the reciprocating piston has a cam surface upon it anda cam follower riding on the cam surface initiates combustion. Otherpreviously used combustion initiating techniques include those where:the piston movement controls the firing gap of the spark device directlyand combustion is initiated when the piston approaches sufficientlyclose to a fixed projection into the power cylinder; where the positionof the power piston is magnetically sensed as it approaches a fixedposition in the cylinder, and combustion is initiated; and where thefixed minimum volume of the power cylinder is sensed by using it as aresonant chamber, and combustion is initiated when this minimum volumeis achieved.

All of these methods have a significant drawback in that combustion isinitiated only when the power piston reaches a fixed point in the powercylinder. If the piston does not reach this fixed point, there is noignition. The piston may not reach this fixed point in the powercylinder for various reasons, including an earlier misfire. Thus, if amisfire occurs and there is not sufficient power developed by the engineto provide the energy necessary to return the power piston to theposition necessary to again initiate combustion, the engine will stopsince combustion will not thereafter occur.

SUMMARY The apparatus according to the present invention solves this andother problems of the prior art by initiating combustion around thepoint in the cycle where the power piston within the engine reversesdirection, without regard to the physical position of the power pistonwithin the power cylinder. That is, combustion may be initiated at asubstantially preselected point in time before the power piston in itscylinder stops and reverses direction, even though the actual physicalposition of the power piston in its cylinder at this point in time mayvary considerably from one piston stroke to another.

This combustion causing apparatus has the advantage that, in the eventof a misfire, the engine will only lose power for a few strokes and thenregain full power. The engine will regain full power in a few strokes,even if there is insufficient energy, immediately after the misfire, tofully return the power piston to its desired normal operating inner deadpoint or top-deadcenter position in the power cylinder, because:immediately before the power piston stops and begins to reversedirection, whatever its linear position in the power cylinder may be,combustion will be initiated; this combustion of gases, although lessefficient as compared to the correct operating point of the engine, willsupply sufficient energy to the power piston to return it to a pointcloser to a desired inner dead point than achieved during the laststroke; and in a succession of these inefficient power strokes thedesired inner dead point of the power piston will again be attained.Thus, in contrast to the prior art, a misfire will not cause the engineto stop and necessitate a restarting.

Briefly, a preferred embodiment of apparatus according to the presentinvention includes means for sensing the instantaneous velocity of apower piston within the free piston engine. The velocity sensing meansincludes a magnetic sensing coil attached to the housing of the freepiston engine and a magnet connected to move with the power piston. Thevoltage output of the magnetic sensing coil is proportional to thevelocity of the power piston and thus an electrical signal output fromthe sensing means is proportional to the power piston velocity. Theelectrical signal output from the sensing means is applied to a Schmitttrigger circuit through a voltage divider network or aresistor-capacitor and diode network, and the Schmitt trigger circuitprovides the properly timed signal to initiate combustion.

The voltage signal output from the sensing means first exceeds thepreset threshold of the Schmitt trigger circuit and that circuitprovides a first signal output which prepares the remaining circuitryfor an ignition initiating signal. When the voltage signal output fromthe sensing means decreases below the preset threshold of the Schmitttrigger circuit, that circuit provides a second signal output which isused to trigger a silicon controlled rectifier of a conventionalcapacitive discharge ignition arrangement and initiate combustion.

Accordingly, it is a primary object of the present invention to provideimproved electrical apparatus for controlling combustion within a freepiston engine in a manner related to the time at which the power pistonwithin the engine reverses direction within its cylinder without thenecessity of regard to the physical position of the power piston withinthe power cylinder.

This and further objects and advantages of the present invention willbecome clearer in the light of the following detailed description of apreferred embodiment of the present invention and from the appendedclaims.

DESCRIPTION OF THE DRAWINGS FIG. 1 shows a diagrammatic view of a freepiston engine and the electrical combustion initiating apparatus of thepresent invention with a velocity probe operated by the free pistonengine providing the electrical signal used to initiate combustion;

FIG. la shows an enlarged view of the velocity probe of FIG. 1;

FIG. 2 shows a schematic representation of electrical circuitryoperating according to the present invention, with an electrical signalfrom the velocity probe applied to a Schmitt trigger circuit; 7 t

FIGS. 3 through 6 show electrical waveforms at various points in theelectrical circuit of FIG. 2;

FIG. 7 shows a schematic representation of an additional electricalcircuit which may be used in conjunction with the electrical circuitryof FIG. 2 to aid in starting a free piston engine;

FIG. 8 shows a diagrammatic representation of a switch shownschematically within FIG. 7;

FIGS. 9 through 12 show waveforms at various points in the electricalcircuit of FIG. 2 when used in conjunction with the electrical circuitof FIG. 7;

FIG. 13 shows a schematic representation of additional electricalcircuitry which may be used in conjunction with the electrical circuitryof FIGS. 2 and 7 to further aid in starting a free piston engine;

FIG. 14 is a circuit diagram similar to the signal generating and inputportion of FIG. 2, but utilizing a preferred resistorcapacitor and diodenetwork with the velocity-responsive transducer to provide an inputsignal for the Schmitt trigger circuit;

FIG. I5 is a graph showing the relationship of the desired ignition timeto the velocity of the power piston and the mean value of the voltageacross the condenser of FIG. 14, during operation of the engine; and

FIG. 16 is a graph showing how the voltage across such condenser variesas the engine starts up from rest.

DESCRIPTION In FIG. 1, a free piston engine generally designated as 13is shown having a power piston 14 and compressor piston 19inter-connected by shaft 21 to form a commonly moveable power means.Power piston 14 is reciprocally moveable within a power cylinder 23formed within a housing 25 of free piston engine 13 to alternatelyexpand and compress gases within a combustion chamber 27 defined withinpower cylinder 23. A permanent magnet generally designated is connectedto shaft 21 to move with power piston 14. A magnet member generallydesignated as is fixed with respect to the housing of free piston engine13 in the embodiment shown. Magnetic member 20 is formed of a coil ofhighly conductive wire wound on magnetic material having a relativelyhigh permeability and a relatively low hysteresis and provides anelectrical signal output across terminals and 32 from that coil. Theelectrical signal is applied to the circuitry of FIG. 2 in a mannerhereinbelow explained. The circuitry of FIG. 2 provides an outputelectrical signal to a conventional spark plug 29 in order to initiatecombustion within the free piston engine 13. Since magnetic member 20 isdesigned to be fixed with respect to the housing of the free pistonengine in the embodiment shown in FIG. 1 and magnet 15 moves with thepower piston 14, magnet 15 and magnetic member 20 reciprocate withrespect to each other within free piston engine 13.

In FIG. 1a, permanent magnet 15 is shown having a first pole l6 and asecond pole l7 interconnected by a portion 18. Magnetic member 20 isshown including a first arm 22 and a second arm 24. The arms 22 and 24of member 20 are attached by a portion 26 so as to override poles 16 and17, respectively, of magnet 15. As can be more clearly seen in FIG. 1a,a coil of wire, designated 28, is wound upon portion 26 of member 20 sothat the changing flux passing from arm 22, through portion 26, and toarm 24 also passes through coil 28 and induces a voltage across theterminals 30 and 32 attached to the ends of the coil 28.

Magnet 15 and member 20 together form a movement-related parametersensing means used with the electronic circuitry of the presentinvention and yield a signal related to the instantaneous velocity of apower piston of a free piston engine, as will be explained hereinafter.The movement-related parameter which is sensed for generating a signalfor the circuitry of the present invention may also be the accelerationof a free piston or the velocity or acceleration of some other movingmass within the engine, a pressure created by a free piston within theengine, a fluid flow produced by the movement of a piston within theengine, the voltage output of a generator run by the free piston engine,or another parameter which is related to the movement of a free pistonor other mass within the engine. Thus, for the purposes of the presentinvention, a movement related parameter is defined to be a parameterwhich gives an indication of the movement of a free piston or other masswithin the engine moving with the free piston, as distinguished from thespecific linear position of the piston or other mass. For the purposesof the present invention, a movement-related parameter is to becontrasted with a displacement parameter which directly indicates theposition or linear displacement of a free piston along its path ofmovement. Examples of such displacement parameters are the displacementsor positions of parts mechanically linked to a moving piston within afree piston engine and the specific displacement or instantaneousposition of a cam follower riding on a cam surface fixed to a movingpiston within the free piston engine.

More broadly, such a movement-related parameter is used to provide acontrolling electrical signal based on a characteristic of operationwhich is reversal-time-related, i.e. which has a predeterminedrelationship to the time at which the power piston ends its compressionstroke and is ready to start its power stroke in the opposite direction.At this reversal point, the piston velocity drops at least momentarilyto zero. The reversal-time-related signal will thus be used to causecombustion at a point in time which has a desired time relationship tothe moment at which the power piston reverses its stroke, even thoughthat moment of reversal may occur at different linear positions of thepower piston from one stroke to another.

In FIG. 2, a variable resistor 34 is connected between terminal 30 andajunction point 36. A resistor 38 is connected between junction point 36and terminal 32, which also functions as the circuit common or ground.Variable resistor 34 and resistor 38 form the two parts of aconventional voltage divider network with the voltage output from thevoltage divider appearing at junction point 36 which is directlyconnected to a further junction point 37. A first diode 40 has its anodeconnected to junction point 37 and its cathode connected to a powerinput terminal 42 which is arranged to connect to a source of positivevoltage. A second diode 44 has its cathode connected to junction point37 andits anode connected to the circuit common which includes terminal32. Diode 40 prevents the voltage at junction point 37 from exceedingthe positive voltage existing on power input terminal 42, and diode 44prevents the voltage at junction point 37 from decreasing below thevoltage at the circuit common. One end of a resistor 46 is alsoconnected tojunction point 37, and the other end of resistor 46 isconnected to a base 48 of an N- P-N transistor generally designated as50. The collector 52 of transistor 50 is connected to power inputterminal 42 through a collector resistor 54. The emitter 56 oftransistor 50 is connected to the circuit common through an emitter biasresistor 58. An additional transistor generally designated 72 includesan emitter 74 also connected to the emitter 56 of transistor 50 and toone end of emitter bias resistor 58. A collector 76 of transistor 72 isconnected to power input means 42 through a collector resistor 78, and abase 80 of transistor 72 is connected to collector 52 of transistor 50through a resistor 82. Transistor 50, its associated bias circuitry, andtransistor 72, along with its associated bias circuitry, form aconventional Schmitt trigger circuit generally designated as 83. A diode84 has its anode connected to the collector 76 of transistor 72 and itscathode connected to the base 86 of a transistor generally designated88. Transistor 88 has an emitter 90 connected to the circuit common anda collector 92 connected to power input means 42 through a collectorresistor 94. A capacitor 96 interconnects the collector 92 of transistor88 and a junction point 98. A diode 100 has its cathode connectedtojunction point 98 and its anode connected to the circuit common toprevent the voltage atjunction point 98 from decreasing below thevoltage of the circuit common.

A pair of input terminals 102 and 104 are also shown in FIG. 2.Terminals 102 and 104 are arranged to connect a conventional 12 to 400volt DC to DC converter 106 to a source of power, not shown, such as 12volt battery. Converter 106 includes a positive output terminal 108 anda negative output terminal 110. A silicon controlled rectifier orthyristor 112 has an anode connected to output terminal 108 and acathode connected to output terminal 110. The gate or trigger of thesilicon controlled rectifier 112 is connected to junction point 98. Acapacitor 114 has one end connected to output terminal 108 and a secondend connected to one end 116 of the primary 118 of a conventionalpulse-high frequency ingition transformer generally designated 120 ofthe type used with capacitor discharge ignitions. The other end 122 ofprimary 118 is connected to the circuit common. The secondary 124 oftransformer 120 has one end 126 connected to the circuit common andanother end 128 arranged to attach to the spark plug 29 initiatingcombustion in the free piston engine or to other known combustioncausing means associated with the free piston engine.

OPERATION In operating the electrical combustion initiating apparatus ofthe present invention, the magnet 15 is attached to move with the powerpiston of the free piston engine, and thus magnet continuallyreciprocates with the ower piston. Member is attached to the housing ofthe free piston engine adjacent the reciprocating magnet 15 such thatthe arms 22, 24 of member 20 are aligned with poles 16, 17 of magnet 15,respectively, as the magnet 15 reciprocates under the member 20. Amagnetic circuit is thus completed from pole 17, through arm 24, throughportion 26, through arm 22, and to pole 16 as the arms 22, 24 overlaythe poles 16, 17. The exact spacing of member 20 and magnet 15 dependsupon the voltage output desired from coil however, a greater spacingallows for variation in magnetic pole strength of magnet 15 from unit tounit without significantly affecting output. The flux traversing member20 passes through coil 28 so as to conventionally induce a voltageacross terminals and 32 attached to the wire ends of coil 28.

Since the rate of change of flux linking coil 28 is arranged to beproportional to the rate of change of overlay between the poles l6.andl7 and the arms 22 and 24, the voltage induced in coil 28 is directlyproportional to the velocity of magnet 15 with respect to coil 28. Thisvoltage induced in coil 28 is represented in ,FIG. 3. Before the arms22, 24 overlay the poles 16, 17, no voltage is induced in coil 28.

As arms 22, 24 begin to overlay poles 16, 17, there is an immediatetraversal of flux from pole 17, through arm 24, through a portion 26,through arm 22, and to pole 16. Depending upon the relative velocitiesof the magnet 15 and the member 20, a step of voltage of a particularamplitude is induced in coil 28. To control the initiation of combustionwithin the free piston engine, the member 20 is placed near the end ofthe stroke of the power piston, also termed the topdead-center or theinner dead point of the power piston. This location will insuregeneration of its desired signals both at the end of a power pistonstroke of normal desired length during operation of the engine and alsoat the end of any power piston stroke of somewhat shorter length whichmight occur during start-up or as a result of a misfire. This is due tothe fact that as shown in FIGS. 1 and 1a, the magnet 15 is substantiallylonger than the core member 20 on which coil 28 is disposed. Thus, overa substantial portion of the piston stroke including the terminalportion, the electromagnetic coupling between member 20 and coil 28 isrelatively constant so that the voltage induced in coil 28 due torelative movement of the member 20 and coil 28 during this portion ofthe piston stroke is independent of the piston position.

Near the end of the power piston stroke the voltage induced in coil 28slowly decreases from the initial positive value of this step to zeroand slowly increases in a negative direction as the power piston andhence magnet 15 lose velocity, stop, and reverse direction in the freepiston engine with respect to coil 28. As poles I6, 17 move from beneatharms 22, 24, again there is no flux traversing member 20 (and hence norate of change of flux) andtthe voltage induced in coil 28 drops to zeroin a step-like fashion.

The induced voltage represented in FIG. 3 is applied to the electricalcircuitry of FIG. 2. The voltage is first reduced by the effect ofvoltage divider resistors 34 and 38 such that a fixed proportion of thevoltage represented in FIG. 3 appears at junction points 36 and 37. Asis well known to those skilled in the electric arts, the exactproportion of the voltage applied to the voltage divider network throughterminals 30 and 32 that will appear at junction point 16 depends on therelative magnitudes of the resistors 34 and 38. In fact, by varyingresistor 34, the precise timing of the initiation of ignition may besomewhat controlled, as will be hereinafter explained.

The fixed proportion of the voltage appearing atjunction point 36 isapplied to Schmitt trigger 83, and in particular to the base 48 of thenormally non-conducting transistor 50. The application of the initialstep of voltage represented in FIG. 3 at T-1 renders the normallynon-conducting transistor 50 conducting by applying a voltage to thebase 48 of transistor 50 which exceeds the voltage at the emitter 56 oftransistor 50 by more than the normally required base-emitter voltage ofa conducting transistor. When transistor 50 begins conducting,

the voltage between the collector 52 and the emitter 56 of transistor 50begins to decrease rapidly towards a collectoremitter saturation voltageof approximately 0.8 volts for a silicon transistor.

Transistor 72 of Schmitt trigger 83 is normally conducting; however,when transistor 50 begins conducting, the base current for transistor 72is shunted through transistor 50 and to the circuit common. Since thebase bias for transistor 72 is shunted through transistor 50, transistor72 conducts less current. Since transistor 72 conducts less current, thecurrent through emitter resistor 58 decreases and the voltage acrossresistor 58 decreases. A decrease in voltage across resistor 58 lowersthe voltage at emitter 56 of transistor 50 and thus increases thevoltage across the base-emitter of transistor 50 which causes transistor50 to become more conducting. The circuit then regenerates in theconventional fashion of a Schmitt trigger until transistor 50 is fullyconducting. When transistor 50 is fully conducting, thecollector-emitter saturation voltage of transistor 50 is approximatelyequal to the necessary base-emitter voltage of transistor 72; thus thereis insufficient voltage remaining across base resistor 82 to providebase current to transistor 72, and transistor 72 is renderednon-conducting.

With transistor 72 in its normal conducting state, the voltage appearingat collector 76 of transistor 72 is just slightly more positive than thecircuit common because of the collector-emitter saturation voltage oftransistor 72 and the current flowing through resistor 58. Whentransistor 50 is rendered conducting and transistor 72 is thus renderednon-conducting, the voltage at collector 76 rapidly rises towards thevoltage of power input terminal 42. This is depicted in FIG. 4 at timeT-l. As the power piston approaches one end of its stroke, and slowstowards zero velocity, the voltage provided from coil 28 decreasestowards zero, as shown in FIG. 3. At time T-2, the voltage at junctionpoint 36 is no longer sufficient to maintain transistor 50 conducting,and the voltage at collector 52 of transistor 50 begins to rise. Thisincrease in voltage at collector 52 begins to render transistor 72conducting, and the circuit again regenerates in the conventionalfashion of a Schmitt trigger until transistor 50 is again non-conductingand transistor 72 is rendered conducting. Thus, at T2 the voltage atcollector 76 of transistor 72 again approaches the voltage level of thecircuit common.

By rendering transistor 50 conducting at T-l and non-conducting at T-2,a pulse of voltage is created at collector 76 of transistor 72. Thispulse of coltage is applied to diode 84 which is inserted toconventionally provide a voltage level shift between collector 76 oftransistor 72 and base 86 of transistor 88. Because of diode 84, thevoltage at base 86 more closely approximates the voltage at the commonterminal of the circuit when transistor 72 is conducting.

Transistor 88 is normally non-conducting since any base current whichmay be supplied to transistor 88 through resistor 78 is shunted throughthe normally conducting transistor 72. However, when transistor 72 isrendered nonconducting at T1 and the voltage at collector 76 oftransistor 72 begins to rise towards the voltage appearing at powerinput terminal 42, transistor 88 is rendered conducting and the voltageat collector 92 of transistor 88 drops from the voltage at power inputterminal 42 to approximately the voltage at the circuit common. At T-2when transistor 72 is again rendered conducting, transistor 88 isrendered non-conducting, and the voltage at collector 92 again risestowards the voltage at power input 42. Thus, transistor 88 has theeffect of inverting the voltage pulse appearing at collector 76 oftransistor 72, as shown in FIG. 5.

Since capacitor 96 is in series with resistor 94 and the internalgate-to-common impedance of silicon controlled rectifier 112, capacitor96 is charged to approximately the voltage at power input terminal 42.As the negative going leading edge occurring at the collector 92 oftransistor 88 at T-1 is applied to capacitor 96, the voltage across itcannot change instantaneously, as is well-known in the electrical arts,and the voltage at junction point 98 attempts to decrease below thevoltage of the circuit common. See FIG. 6. Since diode 100 preventsjunction point 98 from decreasing in voltage below the voltage value ofthe circuit common, at T-l capacitor 96 rapidly discharges through thenow conducting transistor 88 and the forward impedance of diode 100. AtT-2, the capacitor is arranged to be fully discharged. When the positivegoing trailing edge appearing at collector 92 of transistor 88 at T-2 isapplied to capacitor 96, again capacitor 96 cannot instantaneouslychange voltage and the positive going step is applied to junction point98, as shown in FIG. 6.

This positive going step renders diode 100 non-conducting because of itspolarity and provides the trigger current necessary to render siliconcontrolled rectifier 112 conducting.

When silicon controlled rectifier 112 conducts, capacitor 114 and theprimary 118 of ignition transformer 120 are connected in series witheach other, and there is a rapid oscillatory build-up of current inignition transformer 120, as in conventional ignition circuits. Thisrapid increase in current is then applied to the spark plug of the freepiston engine by secondary 124 of ignition transformer 120 in aconventional manner.

It may now be seen that varying the resistance of variable resistor 34of the voltage divider network comprising resistor 34 and resistor 38varies the exact timing of the initiation of combustion by varying thepoint at which transistor 50 changes back from a conducting state to itsnormally non-conducting state. More particularly, if resistor 34 isdecreased in value, a larger proportion of the voltage applied acrossterminals 30 and 32 will appear across resistor 38 and hence be appliedto the input of Schmitt trigger 83. This larger proportion of voltagewill delay the point in time at which the voltage applied to the base 48of transistor 50 will again decrease sufficiently to cause transistor 50to become non-conducting by the regeneration of Schmitt trigger 83.Thus, decreasing the value of variable resistor 34 will delay theinitiation of combustion. Conversely, increasing the value of resistor34 will cause a decreased proportion of the voltage applied terminals 30and 32 to appear across resistor 38, and this decreased voltage willcause transistor 50 to return to its normally nonconducting state at anearlier point in time. Thus decreasing the value of variable resistor 34has the effect of initiating combustion at an earlier point in time. Ineffect, the timing can be varied so that combustion is triggered at adesired time interval before the point of reversal or time of zerovelocity of the power piston. This triggering point occurs when theelectrical signal, which is proportional to the velocity of the powerpiston in this embodiment, has decreased by a predetermined percentagebelow its peak value (T-l in FIG. 3), for example to the value shown atT-2 in FIG. 3.

It has been found that in starting a free piston engine, it is useful tobring the power piston from its bottom dead center position to its topdead center position much more slowly than occurs when the engine isrunning. Since the velocity probe of FIG. 1 is designed to produce theproper output voltage levels qt operating speeds and the voltage levelsare designed to decrease in direct proportion to a decrease in operatingspeed, during the initial starting stroke of the power piston within thefree piston engine, the voltage output provided by coil 28 is verynearly zero. That is, the speed of the power piston as it initiallymoves from its bottom dead center position towards its top dead centerposition in the power cylinder of the free piston engine is on the orderof 1/100 of its maximum normal operational velocity, and therefore therate of change of flux within coil 28 as magnet passes it issubstantially reduced. Thus, during the initial starting stroke of thepower piston within the free piston engine, the output voltage providedby coil 28 is very likely to be insufficient to operate Schmitt trigger83 even if applied directly to base 48 of transistor 50. Moreover,during the first few strokes of the engine, the power piston iscontinually increasing in maximum speed towards the maximum speed whichexists during the normal running condition. During these first fewcycles of operation, the output provided from coil 28 is also reduced inmaximum amplitude. It has been found that the peak value of voltageduring these initial strokes, as divided by resistor 34 and resistor 38,is very likely to be less than the voltage required to operate Schmitttrigger 83 when applied through the voltage divider network, and thusSchmitt trigger 83 may not operate during these first few strokes.

To overcome the problem of the near zero first stroke output of coil 28,the circuitry of FIG. 7 may be used. In FIG. 7, a resistor of a largevalue is shown connected between power input terminal 42 and ajunctionpoint 132. A capacitor 134 is connected between junction point 132 andthe circuit common. The combination of resistor 130 and capacitor 134are chosen to provide an extremely long time constant, on the order ofone-tenth to 1 second. A switch 136 is connected between junction point132 and input terminal 70. A base resistor 66 is connected between inputterminal 70 and a base 68 of a transistor 60. Transistor 60 is connectedin electrical parallel with transistor 50 of FIG. 2 such that thecollector 62 of transistor 60 is connected with collector 52, and theemitter 64 of transistor 60 is connected to the emitter 56 of transistor50. A resistor 139 also has one end connected to input 70 and has itsother end connected to the circuit common. Resistor 139 is chosen to beof a low value in comparison to resistor 130.

In operation, capacitor 134 charges through resistor 130 and stores thevoltage existing at power input terminal 42. During the initial startingstroke of the engine, switch 136 is closed, as will be explainedhereinafter, and the voltage existing upon capacitor 134 is rapidlydischarged through resistor 139. During the time the voltage acrosscapacitor 134 is being rapidly discharged, a positive pulse is appliedto input 70 of transistor 60 as is shown at T-3 in FIG. 9. Sincetransistor 60 is in electrical parallel with transistor 50 and thevoltage pulse provided by discharge of capacitor 134 is of sufficientamplitude to render transistor 60 conducting, Schmitt trigger 83 iscaused to operate as heretofore explained with the temporarysubstitution of transistor 60 for transistor 50. The curves of FIGS. 9to 12 indicate the voltage and time relationships during this phase ofoperation as compared to the corresponding voltage and timerelationships when the engine is running at full speed as represented inFIG. 3 to 6.

The diagrammatic representation of FIG. 8 indicates the manner in whichswitch 136 is closed. Switch 136 is in a form of a commonly availablereed switch having a glass enclosure 140 and magnetically actuatablecontacts 142 and 144. A magnet 146, different from magnet 15 of FIG. 1,is also mounted to reciprocate with the power piston of the free pistonengine sufficiently close to switch 136 to magnetically actuatablecontacts 142 and 144 during a portion of the stroke of the power pistonof the free piston engine. A vane 145, composed of magnetic typematerial, is shown between magnet I46 and switch 136. When the freepiston engine is running at the proper speed and it is desired todeactivate the circuit of FIG. 7, vane 145 may be pneumaticallyinterposed between magnet 146 and switch 136 to prevent magnet 146 fromclosing the contacts 142, 144 of switch 136. Vane 145 is not necessary,however, since the value of resistor 139 is chosen to be small bycomparison to the value of resistor 130 and since the time constant ofresistor 130 and capacitor 134 are chosen to be in excess of 0.1 secondwhile the normal stroke time of the free piston engine is in the orderof 0.001 second. There is thus insufficient time during a stroke forvoltage to build up across capacitor 134 and capacitor 134 is dischargedeach cycle through resistor 139. Thus after the initial stroke of theengine, transistor 60 is not rendered conducting. Vane 145 may beuseful, however, in prolonging the life of switch 136.

It will be realized by those skilled in the art that since the pistonmovement itself closes switch 136 on the initial starting stroke of thepower piston of the free piston engine, the spark will be delayedsomewhat from the time switch 136 is closed. Since the power piston ismoving so slowly on this initial starting stroke, the slight delay ofelectrical circuitry does not significantly affect the operation of theengine.

FIG. 13 illustrates additional electrical circuitry which may be usedwith the circuitry of FIGS. 7 and 8 to further aid in starting the freepiston engine to overcome the problem of the reduced amplitude signalsfrom coil 28 during the first few cycles of operation. In FIG. 13, apair of control input terminals 150 and 152 are shown. A resistor 154 isconnected between control input terminal 150 and junction point 156. Acapacitor 158 is connected between junction point 156 and input terminal152 which is in turn connected to the circuit common. Junction point 156is further connected to a base 160 of a transistor 162. A collector 164of transistor 162 is connected to a junction point 36 of FIG. 2 througha resistor 166. Terminals 150 and 152 are arranged to be connected to anelectrical voltage generated by the engine such that the electricalvoltage is present in full amplitude only after the engine has reachedproper operating speed.

Thus, transistor 162 is non-conducting during starting and resistor 38alone forms one leg or part of the voltage divider network consisting ofresistor 34 and resistor 38 in FIG. 2. A first voltage division ratio isthus provided. After the first few cycles of operation are complete andthe voltage provided by coil 28 has reached its proper amplitude, alesser voltage division ratio may be used. At this point, the electricalvoltage generated by the engine has also reached a proper level torender transistor 162 conducting. When transistor 162 is conducting,resistor l66 is placed in electrical parallel with resistor 38 and thevalue of the resistance of that leg of the voltage divider network isreduced to thus provide the desired lesser voltage division ratio. Thus,in selecting the value of resistor 38, consideration may be given to theproportion required during the first few cycles of operation of theengine. By the proper selection of resistor 166, the voltage divisionratio may be decreased for normal operation of the engine.

Of course, if an electrical output is not available from the free pistonengine, the switching function of transistor 162 may as well beaccomplished by a mechanical switch. Such a switch may be caused tooperate from a damped butterfly valve positioned in the exhaust of thefree piston engine such that when exhaust indicates the engine isoperational, the butterfly valve is caused to open by the passage of theexhaust gases, and the switching function of transistor 162 would bemechanically accomplished.

FIGS. 14 through 16 show details of construction and operation of apreferred resistor-capacitor (RC) and diode network for replacing thevoltage divider network illustrated at the left portion of FIG. 2 anddescribed above. As shown in FIG 14, the coil 28, in which electricalsignals proportional to instantaneous velocity of the power piston aregenerated, is connected at one end 32 to the ground or common circuitthrough parallel paths, one of which includes a condenser 170, and theother of which includes a fixed resistor 171 in series with a variableresistor 172. The other terminal 30 of the velocity sensing coil 28 isconnected through a diode 173 to the input 37 of the Schmitt triggercircuit, which is shown in dotted outline at 83.

As in the circuit of FIG. 2, diodes 40 and 44 are connected between thepositive voltage input source 42 and the junction 37 at the input to theSchmitt trigger circuit, and between the junction 37 and the ground orcommon circuit, respectively. Diodes 40 and 44 provide essentially thesame functions previously described in connection with FIG. 2. Moreover,the diode 44 prevents the application to input terminal 37 of undesiredtransient signals which might otherwise be generated in the RC part ofthe network.

FIG. illustrates the relationships of piston velocity and voltage atvarious points. Thus curve 174 illustrates the instantaneous velocity ofthe piston starting at point A corresponding to the outer reversal ordead point at which the piston velocity is zero just as the pistonreverses to start its compression stroke. The piston velocity thenreaches a maximum at point B relatively rapidly during normal operation,as return energy is supplied to the piston in known manner. As thecompression stroke continues, and the mixture in the combustion chamberis compressed, the piston velocity decreases from its peak value at Band ultimately reaches zero at point C on the graph, corresponding tothe inner reversal point at the end of the compression stroke and thebeginning ofthe power stroke. The piston velocity then increases rapidlyin the other direction during the power stroke, reaching a maximum atpoint D and then again reaching zero at the outer reversal or dead pointof the piston, shown at E on the graph.

During normal operation, the mean value of the voltage on condenser hasan absolute value illustrated by line 176 in FIG. 15 and by its distance178 from the horizontal axis of the graph. The actual voltage signalacross coil 28 will have a value which varies in essentially the samefashion as curve 174, since it is proportional at all times to theinstantaneous piston velocity. When that voltage, which is effectivelyreduced by the negative bias of the voltage across condenser 170, asdescribed below, reaches a maximum corresponding to point !B on curve174 (or some other preselected value corresponding to a point slightlyahead of point B), the Schmitt trigger circuit will be set in onecondition as previously described. When the net effective voltage, asapplied at point 37 (FIGS. 2 and 14), drops a predetermined percentagefrom its peak value (or its slightly lower preselected value), theSchmitt trigger circuit will change to the condition causing a spark inthe engine combustion chamber. This firing point is shown at F along thepath of movement of the piston in FIG. 15. Line 180 in that figure, andits distance 182 above the axis of the graph represent the desiredpredetermined decreased velocity (or velocity-responsive signal) atwhich the Schmitt trigger circuit causes such ignition.

FIG. 16 shows how the mean value of the voltage (shown at 184) acrosscondenser 170 gradually increases in a negative biasing direction as theengine 'starts from rest (at the left of the graph) and reaches normaloperation (at the right of the graph). The actual instantaneous voltageacross condenser 170 will vary as shown by curve 186, depending on thetime constant of the R-C circuit.

When the engine fires initially, as shown at the left end of the graphin FIG. 16, the mean value of the voltage across con denser 170 is zero.The firing point shown at F in FIG. 15 will accordingly be later, inrelation to the reversal point of the piston, i.e. farther from the peakvelocity point B of the piston and closer to reversal point C than thenormal firing point desired during operation. As the engine acceleratesto its steady running condition, the mean value of the voltage oncondenser 170 will increase in a negative sense as shown in FIG. 16.Thus the spark is progressively advanced.

During operation according to the present invention, we have found thatfor optimum running, the spark should occur in some cases when thepiston velocity has dropped only about l0 percent from the peak velocitywhich is developed (see point B in FIG. 15) during its compressionstroke, i.e. substantially ahead of the point at which the pistonvelocity drops to zero at the time of reversal of the piston movement asthe piston starts its power stroke (see point C in FIG. 15). To obtainsuch an early signal from the circuit of FIG. 2 would require arelatively high division ratio in the voltage divider network of thatcircuit and could provide an undesirably low voltage input to theSchmitt trigger circuit. Thus the voltage might be too low to cause aspark. In the circuit of FIG. 14, however, a voltage is developed acrossthe parallel R-C combination which has a mean value proportional to themean engine velocity. This voltage is added to the velocity signal ofcoil 28 in a manner which biases the signal from coil 28 negatively.Thus the voltage across the condenser 170, in effect, is subtracted fromthe voltage signal across coil 28.

Should a misfrre occur, the velocity signal amplitude at coil 28 falls.However, by choosing the time constant of the R-C part of the network inFIG. 14 properly, so that it is equal to or shorter than the mechanicaltime constant of the engine, the negative bias of the Schmitt inputsignal resulting from the mean voltage level across condenser 170 willbe immediately reduced or removed. Thus even the reduced signal fromcoil 28 will be sufficient to insure that the signal applied from point37 to the Schmitt trigger is still sufficient to produce a spark in aneffective time relationship to the moment of reversal at the end of thenext compression stroke after the misfire.

In one embodiment of the circuit of FIG. 14, condenser 170 has a valueof 12 microfareds, while resistor 171 has a value of 2,200 ohms inseries with a potentiometer 172 having a maximum resistance of 10,000ohms.

The operation of the Schmitt trigger circuit in connection with theinput network of FIG. 14 is essentially similar to the operation of thatcircuit as previously described in FIG. 2.

Now that the basic teachings of the present invention have beenexplained, many extensions and variations will be obvious to one skilledin the art. For example, many configurations of the circuitry of apreferred embodiment will be envisioned to perform according to thepresent invention; no limitation to this exact circuitry is intendedAlso, functional blocks other than those shown can be used to performaccording to the present invention. No limitation to the precise blocksis intended.

Further, while the preferred embodiment of the present invention hasbeen explained in relation to spark ignition free piston engines, theprinciples of operation of the present invention may be applied to freepiston Diesel engines. In the case of a Diesel engine, a combustioncausing means may be a fuel injector system and the electrical circuitryof the present invention may control the fuel pump associated with thefuel injector system. Further, the principles of operation of thepresent invention may be applied to a stratified charge free pistonengine where both the spark ignition and the injection systems arecontrolled accordingly. The spark ignition system is preferred; however,no limitation to this particular system is intended.

Furthermore, while the operation of the present invention has beenexplained with the relation to a Schmitt trigger, no limitation to thisprecise circuit is intended. By reference to a Schmitt trigger, anyvoltage level responsive trigger means with hysteresis is intended. Infact, a differential amplifier may be used with threshold referencesignal applied to one input and the voltage from junction point 37 ofFIG. 2 applied to the other input. The Schmitt trigger circuitry has aninherent advantage over a differential amplifier in that, as iswell-known to those skilled in the electrical arts, a Schmitt triggermay be designed with hysteresis. Therefore, with as little as a fewtenths of a volt hysteresis, once the voltage input to the Schmitttrigger circuit 83 has reached an appropriate level to change the stateof the output, noise cannot immediately cause the output to revert toits preceding state. That is, if transistor 50 of FIG. 2 is conductingand the voltage level at base 48 of transistor 50 falls below thevoltage necessary to maintain transistor 50 in a conducting state,transistor 50 is rendered non-conducting and there is a change in theoutput waveform from the Schmitt trigger. Ifa noise pulse followsimmediately and causes the input voltage to again exceed the voltagenecessary to maintain transistor 50 conducting, the hysteresis of theSchmitt trigger will prevent this noise pulse from again changing theoutput waveform. If a differential amplifier were used, this noise pulsewould again cause the output of the differential amplifier to changestates. Since the noise pulse is usually of short duration, the outputfrom the differential amplifier would immediately revert to itsalternate state. Thus, a train of pulses could issue from differentialamplifier due to noise on the incoming waveform where the hysteresis ofa Schmitt trigger would eliminate this effect of the noise.

In summary, the present invention provides improved electrical means forcontrolling the time at which combustion is initiated during the powerpiston movement of a free piston engine, by developing an electricalsignal having a desired reversal-time-related characteristic, i.e. adesired time relationship to the moment in time when the power pistonreaches its reversal point at the end of a compression stroke, eventhough that reversal point may occur at different physical positions orlinear displacements of the piston along its path of movement in theengine. The foregoing specification sets forth the nature and principlesof the invention and some of the ways in which it may be practiced.

Now, therefore, we claim:

1. In a free piston engine having a power cylinder, a movable pistonassembly including a power piston which is freely reciprocable in saidcylinder along a path of movement between desired normal inner and outerpiston reversal points and in which a power stroke in one directiontoward the outer reversal point immediately follows a compression strokein the opposite direction toward the inner reversal point, andcombustion-causing means for initiating combustion in the cylinder tomove the power piston in one direction for each successive power stroke,the improvement comprising electrical sensing means including tworelatively movable electrical members operable upon relative movementthereof to produce an electrical sensing signal dependent in magnitudeupon the speed of such relative movement, one of which electricalmembers is operatively connected to the piston, and which members areelectrically coupled to each other in a relatively constant manner overa substantial portion, including the final portion, of the movement ofthe piston during a compression stroke so that an electrical sensingsignal is produced by said sensing means determined by the instantaneousvalue of a movement-related power piston characteristic which has apredeterminable relationship to the moment when the power piston reachesa reversal point at the end of each compression stroke, and which issubstantially independent of variations in the actual physical positionof the piston along its path at the moment of piston reversal fordifferent power strokes, and electric control circuit means having inputmeans and output means, the input means being connected to receive theelectrical sensing signal from the sensing means, and the output meansbeing connected to operate the combustioncausing means by providing apredetermined output signal at said output means, the electric controlcircuit means also having electric control signal generating meansresponsive to a predetermined electric sensing signal at the input meansfor providing said predetermined output signal at the output means at apoint in time having a desired time relationship to the moment of powerpiston reversal at the end of each compression stroke.

2, The free piston engine apparatus of claim 1 having electrical networkmeans connecting the sensing means to the input means of the electriccontrol circuit means, the sensing means having a coil in which theelectrical sensing signal is generated, said network means including anR-C section having a resistor and a capacitor connected in parallel witheach other and in series with the sensing means coil and also includinga diode, the input means of the electric control circuit means havingtwo input terminals, and said network means having means connecting theR-C section, sensing means coil and diode in series with each otheracross said input terminals.

3. An improved free piston engine apparatus according to claim 1 inwhich the sensing means includes first and second relatively movableparts, one of which is connected to move with the piston assembly, theelectric control signal generating means comprising voltage levelresponsive trigger means for providing a first output control signalwhen the voltage level of the electrical sensing signal applied to theinput means is below a threshold amplitude and providing a second outputcontrol signal when the voltage level of the electrical sensing signalapplied to the input means is above the threshold amplitude, saidelectric control signal generating means providing said predeterminedoutput signal at the output means in response to one of said outputcontrol signals from the trigger means, said means for causingcombustion including control input means for initiating operation of thecombustion causing means when said predetennined output signal isapplied to the control input means, and the output means ofthe electriccontrol circuit means being connected to the control input means of thecombustion causing means.

4. The free piston engine apparatus of claim 3, wherein the voltagelevel responsive trigger means comprises a Schmitt trigger having outputmeans, the signal from the sensing means causing a pulse output from theSchmitt trigger output means.

5. The free piston engine apparatus of claim 4, having voltage dividingmeans connecting the sensing means to the input means of the electriccontrol circuit means, the voltage dividing means comprising: firstresistive means; second resistive means; means for connecting thesensing means, the first resistive means and the second resistive meansin electrical series; voltage divider output means connected across thesecond resistive means; and means for connecting the voltage divideroutput means to the input means of the electric control circuit meansand thereby to the Schmitt trigger.

6. The free piston engine apparatus of claim further including aninverting pulse amplifier having an input means and an output means;means for connecting the input means of the inverting amplifier to theoutput means of the Schmitt trigger; capacitive means for connecting theoutput means of the inverting amplifier to the electric control circuitoutput means and thereby to the control input of the combustioninitiating means; and diode means having one end connected to thecontrol input of the combustion initiating means and having the otherend connected to provide a unidirectional electrical current path to acommon point in the electrical circuitry.

7. The free piston engine apparatus ofclaim 5 also including means foraiding in the starting of the free piston engine, comprising incombination: controlled switch means; further resistive means; andconnection means for connecting the further resistive means andcontrolled switch means in electrical series and for connecting theseries connection of the further resistive means and the controlledswitch means in electrical parallel with the second resistive means forlowering the voltage division ratio of the voltage dividing means whenthe switch means is closed; the controlled switch means being arrangedto be closed upon the normal operation of the free piston engine.

8. The free piston engine apparatus of claim 3 having voltage dividingmeans connecting the sensing means to the input means of the electriccontrol circuit means, the voltage dividing means comprising: firstresistive means; second resistive means; means for connecting thesensing means, the first re sistive means and the second resistive meansin electrical series; voltage divider output means connected across thesecond resistive means; and means for connecting the voltage divideroutput means to the input means ofthe electric control circuit means andthereby to the trigger means.

9. The free piston engine apparatus of claim 8, also including means foraiding in the starting of the free piston engine, comprising incombination: controlled switch means; further resistive means; andconnection means for connecting the further resistive means andcontrolled switch means in electrical series and for connecting theseries connection of the further resistive means and the controlledswitch means in electrical parallel with the second resistive means forlowering the voltage division ratio of the voltage dividing means whenthe switch means is closed; the controlled switch means being arrangedto be closed upon the normal operation of the free piston engine.

10. The free piston engine apparatus of claim 1 including switch meansoperated upon said piston reaching a predetermined position during theinitial starting operation of the engine, means responsive to theoperation of said switch means for causing operation of saidcombustion-causing means independently of the value of the signal fromsaid electrical sensing means, and means effective to prevent saidswitch means from affecting the operation of said combustion-causingmeans during operation of said engine after combustion has been iniiated.

t 11. The free piston engine apparatus of claim I having electricalnetwork means connecting the electrical sensing means to the input meansof the electric control circuit means, the electrical sensing meanshaving as one of said two relatively movable electrical members a coilin which the electrical sensing signal is generated, said network meansincluding means for connecting said coil to said input means of saidelectric circuit control means and also including capacitive meanseffective as said engine speed increases to decrease the effect on saidcircuit control means of a sensing signal of a predetermined magnitudefrom said coil,

1. In a free piston engine having a power cylinder, a movable pistonassembly including a power piston which is freely reciprocable in saidcylinder along a path of movement between desired normal inner and outerpiston reversal points and in which a power stroke in one directiontoward the outer reversal point immediately follows a compression strokein the opposite direction toward the inner reversal point, andcombustion-causing means for initiating combustion in the cylinder tomove the power piston in one direction for each successive power stroke,the improvement comprising electrical sensing means including tworelatively movable electrical members operable upon relative movementthereof to produce an electrical sensing signal dependent in magnitudeupon the speed of such relative movement, one of which electricalmembers is operatively connected to the piston, and which members areelectrically coupled to each other in a relatively constant manner overa substantial portion, including the final portion, of the movement ofthe piston during a compression stroke so that an electrical sensingsignal is produced by said sensing means determined by the instantaneousvalue of a movement-related power piston characteristic which has apredeterminable relationship to the moment when the power piston reachesa reversal point at the end of each compression stroke, and which issubstantially independent of variations in the actual physical positionof the piston along its path at the moment of piston reversal fordifferent power strokes, and electric control circuit means having inputmeans and output means, the input means being connected to receive theelectrical sensing signal from the sensing means, and the output meansbeing connected to operate the combustion-causing means by providing apredetermined output signal at said output means, the electric controlcircuit means also having electric control signal generating meansresponsive to a predetermined electric sensing signal at the input meansfor providing said predetermined output signal at the output means at apoint in time having a desired time relationship to the moment of powerpiston reversal at the end of each compression stroke.
 2. The freepiston engine apparatus of claim 1 having electrical network meansconnecting the sensing means to the input means of the electric Controlcircuit means, the sensing means having a coil in which the electricalsensing signal is generated, said network means including an R-C sectionhaving a resistor and a capacitor connected in parallel with each otherand in series with the sensing means coil and also including a diode,the input means of the electric control circuit means having two inputterminals, and said network means having means connecting the R-Csection, sensing means coil and diode in series with each other acrosssaid input terminals.
 3. An improved free piston engine apparatusaccording to claim 1 in which the sensing means includes first andsecond relatively movable parts, one of which is connected to move withthe piston assembly, the electric control signal generating meanscomprising voltage level responsive trigger means for providing a firstoutput control signal when the voltage level of the electrical sensingsignal applied to the input means is below a threshold amplitude andproviding a second output control signal when the voltage level of theelectrical sensing signal applied to the input means is above thethreshold amplitude, said electric control signal generating meansproviding said predetermined output signal at the output means inresponse to one of said output control signals from the trigger means,said means for causing combustion including control input means forinitiating operation of the combustion causing means when saidpredetermined output signal is applied to the control input means, andthe output means of the electric control circuit means being connectedto the control input means of the combustion causing means.
 4. The freepiston engine apparatus of claim 3, wherein the voltage level responsivetrigger means comprises a Schmitt trigger having output means, thesignal from the sensing means causing a pulse output from the Schmitttrigger output means.
 5. The free piston engine apparatus of claim 4,having voltage dividing means connecting the sensing means to the inputmeans of the electric control circuit means, the voltage dividing meanscomprising: first resistive means; second resistive means; means forconnecting the sensing means, the first resistive means and the secondresistive means in electrical series; voltage divider output meansconnected across the second resistive means; and means for connectingthe voltage divider output means to the input means of the electriccontrol circuit means and thereby to the Schmitt trigger.
 6. The freepiston engine apparatus of claim 5 further including an inverting pulseamplifier having an input means and an output means; means forconnecting the input means of the inverting amplifier to the outputmeans of the Schmitt trigger; capacitive means for connecting the outputmeans of the inverting amplifier to the electric control circuit outputmeans and thereby to the control input of the combustion initiatingmeans; and diode means having one end connected to the control input ofthe combustion initiating means and having the other end connected toprovide a unidirectional electrical current path to a common point inthe electrical circuitry.
 7. The free piston engine apparatus of claim 5also including means for aiding in the starting of the free pistonengine, comprising in combination: controlled switch means; furtherresistive means; and connection means for connecting the furtherresistive means and controlled switch means in electrical series and forconnecting the series connection of the further resistive means and thecontrolled switch means in electrical parallel with the second resistivemeans for lowering the voltage division ratio of the voltage dividingmeans when the switch means is closed; the controlled switch means beingarranged to be closed upon the normal operation of the free pistonengine.
 8. The free piston engine apparatus of claim 3 having voltagedividing means connecting the sensing means to the input means of theelectric control circuit means, thE voltage dividing means comprising:first resistive means; second resistive means; means for connecting thesensing means, the first resistive means and the second resistive meansin electrical series; voltage divider output means connected across thesecond resistive means; and means for connecting the voltage divideroutput means to the input means of the electric control circuit meansand thereby to the trigger means.
 9. The free piston engine apparatus ofclaim 8, also including means for aiding in the starting of the freepiston engine, comprising in combination: controlled switch means;further resistive means; and connection means for connecting the furtherresistive means and controlled switch means in electrical series and forconnecting the series connection of the further resistive means and thecontrolled switch means in electrical parallel with the second resistivemeans for lowering the voltage division ratio of the voltage dividingmeans when the switch means is closed; the controlled switch means beingarranged to be closed upon the normal operation of the free pistonengine.
 10. The free piston engine apparatus of claim 1 including switchmeans operated upon said piston reaching a predetermined position duringthe initial starting operation of the engine, means responsive to theoperation of said switch means for causing operation of saidcombustion-causing means independently of the value of the signal fromsaid electrical sensing means, and means effective to prevent saidswitch means from affecting the operation of said combustion-causingmeans during operation of said engine after combustion has beeninitiated.
 11. The free piston engine apparatus of claim 1 havingelectrical network means connecting the electrical sensing means to theinput means of the electric control circuit means, the electricalsensing means having as one of said two relatively movable electricalmembers a coil in which the electrical sensing signal is generated, saidnetwork means including means for connecting said coil to said inputmeans of said electric circuit control means and also includingcapacitive means effective as said engine speed increases to decreasethe effect on said circuit control means of a sensing signal of apredetermined magnitude from said coil.